Experimental investigation of gas dynamic interactions in laser cutting for nuclear decommissioning

The UK’s Nuclear Decommissioning Authority (NDA) is responsible for Europe’s largest nuclear decommissioning and remediation programme. The programme covers 17 legacy sites across the UK, ranging from research to fuel fabrication and enrichment facilities. Remote laser deployment cutting in harsh environments has been identified as a process that is key to delivering the NDA’s 120- year programme in a safe and timely manner. However, hazardous aerosolised byproducts generated during the laser cutting process are limiting the processing capability of laser cutting as a nuclear decommissioning tool. An improved understanding of the fundamental mechanisms responsible for aerosol formation in laser cutting is required, along with how laser process parameters can be changed to create aerosolised by-products that are easier to manage. For nuclear decommissioning the aim is increase the mean size of aerosolised by-products meaning they place reduced demand filtration systems. A review of the literature highlights that the gas dynamic interactions that take place during laser cutting are key factors in the size and shape of the aerosols generated.

This thesis conducts an in-depth study, in an idealised environment, of the gas dynamic interactions that take place during laser cutting. This study focuses on cutting conditions most likely present during laser nuclear decommissioning using the schlieren imaging technique to visualise changes in gas density. The result of the schlieren imaging work highlights that vastly different gas dynamic characteristics inside the laser cut kerf can be created by changing process variables such as stand-off distance, nozzle diameter, nozzle type and supply pressure. Hence, the results of this study suggest it is possible to influence the breakdown mechanisms and, therefore, aerosol by-product formation in laser cutting through control of cutting parameters. Correlating this data with the pressure experienced on the surface of the workpiece demonstrates that surface pressure plays a governing role in the location of key gas dynamic features present in the laser cut kerf.

To confirm that changing key gas dynamic features inside the laser cut kerf do influence the breakdown mechanism and hence the size and volume of aerosolised byproducts produced, high speed, high magnification imaging of the melt being ejected during laser cutting was conducted. The imaging showed that reducing the surface pressure, whilst maintaining the ability to cut, significantly increases the average size of imaged particles. In addition, the volume of material that remained adhered to the underside of the workpiece also increased with decreasing assist gas supply pressure, with no discernible change in kerf width. This enables the conclusion that the total volume of aerosolised by-products is also reduced. Imaging of melt ejection also enabled a comparison between a reactive gas (air) and a non-reactive gas (nitrogen).

The two gases showed vastly different melt ejection regimes, with the reactive gas (air) being much more violet, producing smaller, faster-moving particles. Estimation of the melt temperature under the two cutting conditions highlighted that the increase in melt temperature and, therefore, the reduction in melt viscosity for the reactive gas was likely responsible for the violent nature of the recorded melt regimes. Estimation of the gas jet properties inside the laser cut kerf indicates the Reynolds number of the gas jet boundary layer at the point at which it separates from the molten cut front is not influenced by material thickness. Hence, predictions of the location of the separation point could be determined for any material thickness based on known input conditions.

Finally, analysis of captured aerosols using an optical particle sizer confirmed that controlling key gas dynamic features in laser cutting can reduce the total number of aerosolised by-products by 90% and reduce the fine particle fraction by 2.6%. The findings of this thesis have highlighted the key features for consideration when understanding melt atomisation in laser cutting and how these key features can be controlled to improve the by-products generated during laser cutting. The findings of this thesis should be considered for helping unlock the potential of laser cutting as a routine nuclear decommissioning tool.